The present invention relates to an ultrasonic probe used in the field of an ultrasonic apparatus for extracting an image of the inner part of an object to be examined by means of ultrasonic beams and particularly to the technique for obtaining a satisfactory ultrasonic image.
An ultrasonic diagnostic apparatus employs an ultrasonic probe to transmit ultrasonic beams to the inner part of an object to be examined and receive an echo signal from the inner part of the object. As the ultrasonic probe, there are one having an array transducer composed of a large number of elongate rod-like elements and another having a single disk-shaped element transducer. The former is largely used in a so-called electronic scanning type ultrasonic apparatus and the latter is chiefly used in a mechanical scanning type ultrasonic apparatus.
In the electronic scanning type ultrasonic apparatus, a plurality of elongate rod-like elements are constituted as one group and the respective elements in the group are given predetermined delay times, respectively, so that each element is driven with the predetermined delay time given thereto. Thus, the probe transmits an ultrasonic beam focused at a predetermined depth in a predetermined direction within the object to be examined. Further, upon reception of the echo signal, the respective elements are also given delay times varying with time to thereby receive an ultrasonic beam from a predetermined direction. The ultrasonic beam for transmission and reception is moved in the azimuth direction of the elements to scan the inner part of the object, so that ultrasonic image data are obtained.
In order to obtain a satisfactory ultrasonic image by the scanning, it is necessary to form the narrow ultrasonic beam which has the excellent directivity over the whole scanning range of the beam. For this purpose, it is important that the acoustic crosstalk between adjacent elements is small.
The basic structure of the ultrasonic probe generally includes an acoustic absorption backing material, a piezoelectric element, an acoustic matching layer and an acoustic lens which are laminated in order. In order to reduce the acoustic crosstalk between the adjacent elements, that is, in order to improve isolation between the adjacent elements, the element is cut together with the acoustic matching layer deeply to the degree that gaps are formed in the acoustic absorption backing material, so that the cut elements are separated from each other. The gaps are filled with polymer resin in order to prevent the elements from being damaged when external force is exerted to the elements. In other words, the conventional ultrasonic probe is manufactured by way of the process in which the piezoelectric element and the acoustic matching layer are first fixed on the acoustic absorption backing material and are then cut by a dicing saw to form an array of the piezoelectric elements. Accordingly, in the ultrasonic probe using the conventional manufacturing method, it is a matter of course that the width of the array of the piezoelectric elements is equal to that of the acoustic matching layer.
Accordingly, in the conventional probe, area for transmission and reception of the element is identical with that of the acoustic matching layer. Further, in the prior art, gaps between the piezoelectric elements are also filled with polymer resin, while the filling of polymer resin is made after cutting the acoustic matching layer and the piezoelectric material with electrodes simultaneously, in order to prevent damage of the piezoelectric elements when external force exceeding forecasted force in the diagnosis is exerted to the elements and accordingly both of gaps between the piezoelectric elements and between the acoustic matching layer elements are filled with the polymer resin.
Recently, the array of the piezoelectric elements of the ultrasonic probe tend to be made small and densified to form the high density structure in order to increase the lateral resolution of an image. Therefore, the width of the array element is as narrow as about 0.2 mm. On the other hand, since the thickness of the piezoelectric element is determined by a frequency or a wavelength .lambda. (actually .lambda./2) of ultrasonic beams, the thickness of the piezoelectric element of the probe is, for example, about 0.44 mm for a frequency of ultrasonic beams of 3.5 MHz and about 0.6 mm for a frequency of 2.5 MHz. Accordingly, as the frequency of ultrasonic beams is low, a ratio of the thickness and the width of the piezoelectric element is large. When the piezoelectric elements are made small for the high density structure, the following problems occur.
More particularly, since miniaturization of the piezoelectric elements for attainment of the high density structure is to narrow the width of the element, it is meant that energy of ultrasonic beams transmitted by a single piezoelectric element and energy of ultrasonic beams received by the single piezoelectric element are reduced as compared with the prior art. That is, the sensitivity of the piezoelectric element is reduced. When the reduction of the sensitivity is to be compensated by a receiving circuit, noise is introduced by an amplifier of a signal and a complicated circuit for preventing the introduction of noise is required.
Further, when the piezoelectric elements having the width of 0.2 mm as described above are formed, the width of the gaps between the piezoelectric elements is as narrow as about 0.075 mm. It is extremely difficult as compared with the prior art to form the gaps having such a width by means of a dicing saw while the acoustic matching layer and the piezoelectric material are adhered to each other as in the prior art. Since the depth of the gaps must be made deeper as the frequency of ultrasonic beams is lower, the difficulty to form the gaps is increased.
Furthermore, since the width of the elements is very narrow and the strength thereof is reduced due to the high density structure of the piezoelectric elements, gaps between the elements are filled with polymer resin to increase the strength, while there is a problem that acoustic crosstalk is produced between adjacent piezoelectric elements through the polymer resin when the elements are driven. In addition, since the width of the gaps relative to the width of the piezoelectric elements is increased, a problem concerning the grating lobe also remains.
On the other hand, attainment of the broad bandwidth is also treated as a problem heretofore apart from the high density structure of the piezoelectric elements, while any methods for solving this problem are not found at the present time and presentation of some solving methods thereof is desired.
It is an object of the present invention to solve the above problems by providing an ultrasonic probe capable of obtaining a satisfactory ultrasonic image.